1. Field of the Invention
The invention relates to a wire guide roll for use in wire saws for simultaneously slicing a multiplicity of wafers from a cylindrical workpiece, in particular from a workpiece consisting of semiconductor material, wherein with the aid of a forward feed device, the workpiece and a wire gang of a wire saw execute a relative movement directed perpendicularly to the longitudinal axis of the workpiece, by which the workpiece is guided through the wire gang.
2. Background Art
Semiconductor wafers are generally produced by slicing a cylindrical mono- or polycrystalline workpiece of the semiconductor material simultaneously into a multiplicity of semiconductor wafers in one working step with the aid of a wire saw.
The essential components of these wire saws include a machine frame, a forward feed device, and a sawing tool which consists of a gang of parallel wire sections. The workpiece is fixed on a so-called sawing strip, generally by cementing or adhesive bonding. The sawing strip is in turn fastened on a mounting plate, in order to clamp the workpiece in the wire saw.
The wire gang of the wire saw is generally formed by a multiplicity of parallel wire sections, which are tensioned between at least two wire guide rolls, the wire guide rolls being rotatably mounted and at least one of them being driven. The wire sections generally belong to a single finite wire, which is guided spirally around the roll system and is unwound from a stock roll onto a receiver roll.
During the sawing process, the forward feed device induces a relative movement of the wire sections and the workpiece directed against one another. As a result of this forward feed movement, the wire, on which a sawing suspension is applied, works to form parallel sawing kerfs through the workpiece. The sawing suspension, which is also referred to as a slurry, contains hard material particles, for example of silicon carbide, which are suspended in a liquid. A sawing wire with firmly bound abrasive may also be used. In this case, it is not necessary to apply a sawing suspension; it is merely necessary to supply a liquid cooling lubricant (for example water), which protects the wire and the workpiece against overheating and at the same time transports workpiece swarf away from the cutting grooves.
The production of semiconductor wafers from cylindrical semiconductor material, for example from single-crystal ingots, places stringent requirements on the sawing method. The purpose of the sawing method is generally that each sawn semiconductor wafer should have two surfaces which are as planar as possible and are mutually parallel.
Besides thickness variation, the planarity of the two surfaces of the semiconductor wafer is of great importance. After a wire saw has been used to slice a semiconductor single crystal, for example a silicon single crystal, the wafers thereby produced have a wavy surface. This waviness may be partially or fully removed in the subsequent steps, for example grinding or lapping, depending on the wavelength and amplitude of the waviness as well as the depth of the material removal. In the least favorable case, such surface irregularities (undulations, waviness), which may have periodicities of from a few mm up to for example 50 mm, may still be detected even after polishing on the finished semiconductor wafer, where they have a detrimental effect on the local geometry.
The parameters “bow” and “warp”, as measures of the deviation of the actual wafer shape from the desired ideal wafer shape (or “sori”) depend crucially on the straightness of the cut, which is in turn crucially determined by properties of the wire guide rolls being used.
The wire guide rolls are conventionally provided with a coating. They furthermore comprise a multiplicity of grooves through which the sawing wire is guided, and by means of which the wire gang of the wire saw is formed.
Wire guide rolls which comprise a polyurethane coating are conventionally used. Polyurethanes are substantially resistant to the abrasive slurry, or to abrasion due to cutting wires with bound abrasive.
Wear of the wire guide rolls, which essentially leads to modification of the geometry of the grooves, nevertheless gradually takes place in the prior art, for example after 50 operations. This, however, is undesirable since optimal guiding of the sawing wire is no longer ensured owing to such modification of the groove geometry.
JP 2006102917 A2 proposes to use urethane with 5-30% by weight of silicon carbide abrasive as a coating, so that the coating is significantly harder than a coating consisting only of urethane. With too hard a wire guide coating, however, the frictional locking between the wire and the wire guide roll is no longer ensured so that the wire gang can no longer be set optimally in motion.
This also applies for the wire guide rolls disclosed in JP 11099465 A2. Here again, the coating of the rolls is intended to consist of a material which has a hardness comparable to abrasive silicon carbide particles. Silicon carbide particles have a hardness of 3000-4000 HV (Vickers hardness).
JP 11262853 adopts a similar approach, by treating the surfaces of the wire guide rolls with low-oxygen plasma. This is intended to make the surfaces of the wire guide rolls more uniform, which is meant to give a wire guide roll lifetime about two times as long as the untreated roll. The waviness of the sawn wafers could also thereby be improved. The wire guide rolls comprise hard urethane resin surfaces. The grooves of unworn wire guide rolls have a V-shaped form which is conventional in the prior art.